Wind-power generation system and operation control method therefor

ABSTRACT

An object is to improve the accuracy of power factor adjustment. Power-factor command values corresponding to individual wind turbines are determined by correcting a predetermined power-factor command value for an interconnection node using power factor correction levels set for the individual wind turbines.

RELATED APPLICATIONS

The present application is National Phase of International ApplicationNo. PCT/JP2007/074121 filed Dec. 14, 2007, the disclosure of which ishereby incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present invention relates to wind-power generation systems andoperation control methods therefor.

BACKGROUND ART

In power factor control at an interconnection node of a wind farm,conventionally, a predetermined power-factor command value is determinedby, for example, consultation with the grid operator so as to satisfythe range of, for example, a leading power factor of 0.95 to a laggingpower factor of 0.95, and generation systems of individual wind turbinesperform power factor control so as to maintain the determinedpredetermined power-factor command. In addition, if the power factor atthe interconnection node deviates from the above range despite suchcontrol, the power factor at the interconnection node is adjusted by theopening/closing of a capacitor bank or reactors at a substation.

In addition, Patent Citation 1 discloses that a central controller forcontrolling the power at the interconnection node and so on transmits auniform reactive power command to the individual wind turbines so thatthe individual wind turbines perform control based on the reactive powercommand.

Patent Citation 1:

U.S. Pat. No. 7,166,928, specification

DISCLOSURE OF INVENTION

To increase the voltage stability of a utility grid, the accuracy ofpower factor adjustment at an interconnection node must be improved. Theconventional technique described above, however, is disadvantageous inthat the accuracy of power factor adjustment cannot be further improvedbecause a uniform reactive power command value is provided to theindividual wind turbines.

An object of the present invention is to provide a wind-power generationsystem and an operation control method therefor in which the accuracy ofpower factor adjustment can be improved.

To solve the above problem, the present invention employs the followingsolutions.

A first aspect of the present invention is an operation control methodfor a wind-power generation system including a plurality of windturbines and a central controller for providing control commands to theindividual wind turbines, and output powers of the individual windturbines are supplied to a utility grid via a common interconnectionnode. Power-factor command values corresponding to the individual windturbines are determined by correcting a predetermined power-factorcommand value for the interconnection node using power factor correctionlevels set for the individual wind turbines.

According to the present invention, because the predeterminedpower-factor command value for the interconnection node is correctedusing the power factor correction levels corresponding to the individualwind turbines, different power-factor command values can be set for theindividual wind turbines. This allows power factor control of theindividual wind turbines based on appropriate power-factor commandvalues taking into account, for example, the properties related to theindividual wind turbines, thus improving the accuracy of power factorcontrol at the grid node.

In the above operation control method for the wind-power generationsystem, the power factor correction levels may be determined based onreactance components present between the individual wind turbines andthe interconnection node.

Thus, because the power-factor command values for the individual windturbines are determined using the power factor correction levels takinginto account the reactance components present between the wind turbinesand the interconnection node, the actual power factor at theinterconnection node can be efficiently adjusted to the predeterminedpower-factor command value.

For example, if simple feedback control is performed to adjust theactual power factor at the interconnection node to the power-factorcommand value without taking into account the reactance componentspresent between the individual wind turbines and the interconnectionnode, it is possible to adjust the power factors at output ends ofgeneration systems of the individual wind turbines to the power-factorcommand value provided to the individual wind turbines, although it isdifficult to adjust the power factor at the interconnection node to thepredetermined power-factor command value. This is because the powerfactor varies depending on, for example, the reactances of power linesconnecting the output ends of the wind turbines to the interconnectionnode. In this respect, according to the present invention, the powerfactor at the interconnection node can be controlled efficiently andaccurately because the individual wind turbines are controlled based onthe power-factor command values taking into account the reactancecomponents present between the individual wind turbines and theinterconnection node.

A second aspect of the present invention is an operation control methodfor a wind-power generation system including a plurality of windturbines and a central controller for providing control commands to theindividual wind turbines, and outputs of the individual wind turbinesare supplied to a utility grid via a common interconnection node. If theplurality of wind turbines include both variable-speed wind turbines andfixed-speed wind turbines, the overall power factor of the fixed-speedwind turbines at the interconnection node is calculated, the differencebetween the calculated power factor and a predetermined power-factorcommand value for the interconnection node is calculated, thepredetermined power-factor command value is corrected using thecalculated difference, and power-factor command values for theindividual variable-speed wind turbines are determined based on thecorrected predetermined power-factor command value.

According to the above method, because the power-factor command valuesfor the variable-speed wind turbines are determined by taking intoaccount variations in power factor due to the fixed-speed wind turbines,the variations in power factor due to the fixed-speed wind turbines canbe absorbed by power factor control of the variable-speed wind turbines.This improves the accuracy of power factor control at theinterconnection node even if fixed-speed wind turbines andvariable-speed wind turbines are both present.

In the above operation control method for the wind-power generationsystem, the power-factor command values corresponding to the individualvariable-speed wind turbines may be determined by correcting thecorrected predetermined power-factor command value using power factorcorrection levels set for the individual variable-speed wind turbines.

Thus, because the power-factor command values for the individualvariable-speed wind turbines are determined by further correcting thepredetermined power-factor command value for the interconnection node,corrected by taking into account the variations in the power factors ofthe fixed-speed wind turbines, using the power factor correction levelsset for the individual variable-speed wind turbines, differentpower-factor command values can be set for the individual variable-speedwind turbines. This allows power factor control of the individualvariable-speed wind turbines based on appropriate power-factor commandvalues taking into account, for example, the properties related to theindividual variable-speed wind turbines, thus further improving theaccuracy of power factor control at the interconnection node.

In the above operation control method for the wind-power generationsystem, the power factor correction levels corresponding to theindividual variable-speed wind turbines may be determined based onreactance components present between the individual variable-speed windturbines and the interconnection node.

Thus, because the power-factor command values for the individualvariable-speed wind turbines are determined by taking into account thereactance components present between the wind turbines and theinterconnection node, the actual power factor at the interconnectionnode can be efficiently adjusted to the power-factor command value.

A third aspect of the present invention is a wind-power generationsystem including a plurality of wind turbines and a central controllerfor providing control commands to the individual wind turbines, andoutput powers of the individual wind turbines are supplied to a utilitygrid via a common interconnection node. Power-factor command valuescorresponding to the individual wind turbines are determined bycorrecting a predetermined power-factor command value for theinterconnection node using power factor correction levels set for theindividual wind turbines.

A fourth aspect of the present invention is a wind-power generationsystem including a plurality of wind turbines and a central controllerfor providing control commands to the individual wind turbines, andoutputs of the individual wind turbines are supplied to a utility gridvia a common interconnection node. If the plurality of wind turbinesinclude both variable-speed wind turbines and fixed-speed wind turbines,the central controller calculates the overall power factor of thefixed-speed wind turbines at the interconnection node, calculates thedifference between the calculated power factor and a predeterminedpower-factor command value for the interconnection node, corrects thepredetermined power-factor command value using the calculateddifference, and determines power-factor command values for theindividual variable-speed wind turbines based on the correctedpredetermined power-factor command value.

The present invention provides the advantage of improving the accuracyof power factor adjustment.

BRIEF DESCRIPTION OF DRAWINGS

[FIG. 1] FIG. 1 is a diagram showing the entire configuration of awind-power generation system according to a first embodiment of thepresent invention.

[FIG. 2] FIG. 2 is a diagram illustrating power factor correction levelsaccording to the first embodiment of the present invention.

[FIG. 3] FIG. 3 is a flowchart showing a procedure of an operationcontrol method for the wind-power generation system according to thefirst embodiment of the present invention.

[FIG. 4] FIG. 4 is a diagram illustrating an operation control methodfor a wind-power generation system according to a second embodiment ofthe present invention.

EXPLANATION OF REFERENCE

-   1: wind-power generation system-   10: central controller-   20: generation system-   30: power line-   WTG1, WTG2, WTGn: wind turbine

BEST MODE FOR CARRYING OUT THE INVENTION

Individual embodiments of wind-power generation systems and operationcontrol methods therefor according to the present invention will bedescribed below with reference to the drawings.

First Embodiment

FIG. 1 is a block diagram showing the entire configuration of awind-power generation system according to this embodiment. As shown inFIG. 1, a wind-power generation system 1 includes a plurality of windturbines WTG1, WTG2, . . . , WTGn (hereinafter denoted simply by thereference sign “WTG” when all wind turbines are referred to and denotedby the reference signs “WTG1”, “WTG2”, etc. when the individual windturbines are referred to) and a central controller 10 for providingcontrol commands to the individual wind turbines WTG. In thisembodiment, all wind turbines WTG are variable-speed wind turbines.

Each wind turbine WTG includes a generation system 20. The generationsystem 20 includes, as the main configuration thereof, for example, agenerator, a variable-frequency converter excitation system capable ofcontrolling the active power and the reactive power of the generator,and a wind turbine controller for providing a power command value to thevariable-frequency converter excitation system.

The powers output from the generation systems 20 of the individual windturbines are supplied through respective power lines 30 to a utilitygrid via a common interconnection node A.

The central controller 10 sets a power-factor command value for theinterconnection node A based on a requested-power-factor command for theinterconnection node A provided from a power management room managinggrid power (for example, an electric utility). The power-factor commandvalue is corrected using power factor correction levels set for theindividual wind turbines WTG1, WTG2, . . . , WTGn, and the correctedpower-factor command values are transmitted to the respective windturbines. Here the details of the power factor correction levels set forthe individual wind turbines will be described later.

The generation system 20 of each wind turbine WTG1, WTG2, . . . , WTGnsets an active-power command value and a reactive-power command value soas to satisfy the power-factor command value provided from the centralcontroller 10. Specifically, the wind turbine controller of thegeneration system 20 monitors the rotational speed of the generator toset an active-power command value corresponding to the rotational speed.In addition, a reactive-power command value satisfying the power-factorcommand value is determined from the active-power command value and therelational expression shown in equation (1) below. At this time, thewind turbine controller sets the reactive-power command value within theoperating range depending on thermal constraints and voltagelimitations. In addition, if the power-factor command is given priority,the setting may be such that the necessary reactive power is supplied byreducing the active power.

$\begin{matrix}\left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack & \; \\{{{Power}\mspace{14mu}{factor}} = \frac{P}{\sqrt{P^{2} + Q^{2}}}} & (1)\end{matrix}$

In equation (1) above, P is the active power, and Q is the reactivepower.

The wind turbine controller provides the set active-power command valueand the set reactive-power command value to the variable-frequencyconverter excitation system. The variable-frequency converter excitationsystem controls the generator based on the active-power command valueand the reactive-power command value provided from the wind turbinecontroller.

With the above power factor control, active powers and reactive powerssatisfying the power-factor command values provided to the individualwind turbines are output from the respective wind turbines WTG and aresupplied to the common interconnection node A through the power lines30.

Next, the power factor correction levels set for the individual windturbines WTG1, WTG2, . . . , WTGn described above will be described indetail.

The above power factor correction levels are determined based onreactance components present between the individual wind turbines WTG1,WTG2, . . . , WTGn and the interconnection node A. In a wind farm havingmany wind turbines, for example, the lengths of the power lines 30connecting the individual wind turbines WTG1, WTG2, . . . , WTGn and theinterconnection node A differ largely. Accordingly, the powers outputfrom the wind turbines are affected by the reactances corresponding tothe distances over the respective power lines 30 before reaching theinterconnection node A.

As a result, for example, if a uniform power-factor command value isprovided to the individual wind turbines, variations in the reactivepower at the interconnection node A can occur and decrease the powerfactor accuracy. In that respect, this embodiment takes into account thepower variations, described above, due to the reactance components ofthe power lines 30 to correct the power-factor command value provided tothe individual wind turbines using the power factor correction levelscorresponding to the individual wind turbines, more specifically, thereactance components of the power lines 30 connecting the individualwind turbines and the interconnection node A.

First, as shown in FIG. 2, let the powers at the output ends of theindividual wind turbines WTG1, WTG2, . . . , WTGi, . . . , WTGn beP₁+jQ₁, P₂+jQ₂, . . . , P_(i)+jQ_(i), . . . , P_(n)+jQ_(n),respectively. In addition, let the reactances of the power lines betweenthe individual wind turbines WTG1, WTG2, . . . , WTGn and theinterconnection node A be jx₁, jx₂, . . . , jx_(i), . . . , jx_(n),respectively, and the powers of the individual wind turbines at theinterconnection node A are defined as P₁′+jQ₁′, P₂′+jQ₂′, . . . ,P_(i)′+jQ_(i)′, . . . , P_(n)′+jQ_(n)′, respectively.

Next, power flow calculation is performed for each wind turbine. Herethe i-th wind turbine will be described as an example. For convenience,let interconnection node voltage V_(grid) =1 pu and phase angleδ_(grid)=0. In addition, let the direction from each wind turbine towardthe interconnection node A be positive in sign for both the active powerP and the reactive power Q. The sign of the power factor alsocorresponds thereto; for example, power factor pf >0 if P >0 and Q >0,and power factor pf <0 if P >0 and Q <0.

Under such conditions, the active power P_(I) and the reactive powerQ_(i) at the output end of the wind turbine WTGi and the active powerP_(i)′ and the reactive power Q_(i)′ at the interconnection node A arerepresented, respectively, as follows.

$\begin{matrix}\left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack & \; \\\left. \begin{matrix}{{P_{i} = {\frac{1}{x_{i}}V_{i}\sin\;\delta_{i}}}\mspace{110mu}} \\{{P_{i}^{\prime} = {\frac{1}{x_{i}}V_{i}\sin\;\delta_{i}}}\mspace{110mu}} \\{Q_{i} = {{{+ \frac{1}{x_{i}}}V_{i}^{2}} - {\frac{1}{x_{i}}V_{i}\cos\;\delta_{i}}}} \\{{Q_{i}^{\prime} = {{{- \frac{1}{x_{i}}}V_{i}\;\cos\;\delta_{i}} + \frac{1}{x_{i}}}}\mspace{31mu}}\end{matrix} \right\} & (2)\end{matrix}$

In this power flow calculation, the active powers have the same value,namely, P_(i)=P_(i)′, because only the reactance components of the powerlines 30 are taken into account. Letting P_(i) and Q_(i) be known,P_(i)′ and Q_(i)′ can be solved from equation (2) above.

As P_(i) and Q_(i), appropriate values (for example, averages) are setby, for example, acquiring the active power P_(i) and the reactive powerQ_(i) at the output end of the wind turbine over a past predeterminedperiod of time (for example, one month, three months, or one year) andanalyzing the acquired data.

The power factor pf_(i) at the output end of the wind turbine isrepresented by equation (3) below, and the power factor pf_(i)′ at theinterconnection node A is represented by equation (4) below.

$\begin{matrix}\left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack & \; \\{{pf}_{i} = \frac{P_{i}}{\sqrt{P_{i}^{2} + Q_{i}^{2}}}} & (3) \\{{pf}_{i}^{\prime} = \frac{P_{i}^{\prime}}{\sqrt{P_{i}^{\prime 2} + Q_{i}^{\prime 2}}}} & (4)\end{matrix}$

As a result, the power factor correction level Δpf_(i) for the i-th windturbine can be determined by equation (5) below:Δpf _(i) =pf _(grid) −pf _(i)′  (5)

In equation (5) above, pf_(grid) is the power-factor command value forthe interconnection node A.

The power factor correction levels Δpf_(i) determined for the individualwind turbines by the method described above are stored in a memory ofthe central controller 10 in association with the respective windturbines and are used for correction of the power-factor command valuein the operation of the wind turbines.

The above power factor correction levels stored in the memory may beupdated, for example, at predetermined time intervals (for example,every one year or three months). For updating, the active powers P_(i)and the reactive powers Q_(i) of the wind turbines may be set toappropriate values (for example, as described above, set using theanalytical results of data over a past predetermined period of time),and these values may be substituted into the above equations to updatethe power factor correction levels for the individual wind turbines.

Next, an operation control method for the wind-power generation systemhaving the above configuration will be described.

First, upon acquisition of the power-factor command value pf_(grid) forthe interconnection node (Step SA1 in FIG. 3), the central controller 10reads the power factor correction levels Δpf_(i) corresponding to theindividual wind turbines WTG1, WTG2, . . . , WTGn from the memory andcorrects the power-factor command value pf_(grid) using the power factorcorrection levels Δpf_(i) (Step SA2). The corrected power-factor commandvalues pf_(i) (=pf_(grid)+Δpf_(i)) are transmitted to the respectivewind turbines WTG1, WTG2, . . . , WTGn (Step SA3).

The wind turbine controllers of the individual wind turbines WTG1, WTG2,. . . , WTGn set active-power command values and reactive-power commandvalues so as to satisfy the respective power-factor command values Δpf₁,Δpf₂, . . . , Δpf_(i), . . . , Δpf_(n) received from the centralcontroller 10 and provide the set active-power command values and theset reactive-power command values to the variable-frequency converterexcitation systems. The variable-frequency converter excitation systemscontrol the generators based on the provided active-power command valuesand the provided reactive-power command values. Thus, the active powersand the reactive powers satisfying the power-factor command valuescorresponding to the individual wind turbines are output from therespective wind turbines and are supplied to the common interconnectionnode A through the power lines 30.

The central controller 10 detects the reactive power and the activepower at the interconnection node A to calculate the actual power factorpf_(grid)′ from the detected values. The difference between thecalculated actual power factor pf_(grid)′ and the power-factor commandvalue pf_(grid) is then calculated, and new power-factor command valuesare calculated so as to offset that difference and are provided as thenext power-factor command values to the respective wind turbines (StepSA4).

The new power-factor command values are determined by further adding thedifference between the actual power factor pf_(grid)′ and thepower-factor command value pf_(grid) and the power factor correctionlevels Δpf_(i) to the power-factor command value pf_(grid), as inequation (6) below.pf _(i) =pf _(grid) +Δpf _(i)+(pf _(grid) −pf _(grid)′)  (6)

The power-factor command values corresponding to the individual windturbines may be calculated thereafter by detecting the actual powerfactor at the interconnection node A at predetermined time intervals andsubstituting the difference in power factor determined from thedetection results, namely, Δpf_(grid)=pf_(grid)−pf_(grid)′, and thepower factor correction levels Δpf_(i) into equation (6) above.

In this way, feedback control can be performed to stabilize the powerfactor at the interconnection node A.

As described above, because the wind-power generation system 1 and theoperation control method therefor according to this embodiment determinepower-factor command values appropriate for the individual wind turbinesby correcting the power-factor command value for the interconnectionnode A using the power factor correction levels corresponding to thereactances present between the individual wind turbines and theinterconnection node A, power factor control taking into account thereactances related to the power lines 30 can be performed in theindividual wind turbines. This improves the accuracy of power factorcontrol at the interconnection node A.

Second Embodiment

Next, a second embodiment of the present invention will be describedusing FIG. 4.

While the case where all wind turbines are variable-speed wind turbineshas been described in the first embodiment described above, the casewhere some wind turbines are fixed-speed wind turbines will be describedin this embodiment.

A wind-power generation system according to this embodiment includes atleast one fixed-speed wind turbine and at least one variable-speed windturbine. As shown in FIG. 4, for example, the first to i-th windturbines are variable-speed wind turbines, whereas the i+1-th to n-thwind turbines are fixed-speed wind turbines. In this case, first, theactive powers P_(i)′ and the reactive powers Q_(i)′ at theinterconnection node A are determined by power flow calculation based onthe same procedure as in the first embodiment described above.

Subsequently, the sums of the active powers and the reactive powers ofthe fixed-speed wind turbines alone at the interconnection node A aredetermined as shown in equations (7) and (8) below.

$\begin{matrix}\left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack & \; \\{P_{fix}^{\prime} = {\sum\limits_{i = {i + 1}}^{n}P_{i}^{\prime}}} & (7) \\{Q_{fix}^{\prime} = {\sum\limits_{i = {i + 1}}^{n}Q_{i}^{\prime}}} & (8)\end{matrix}$

Subsequently, the above sums of the active powers and the reactivepowers are used to calculate the overall power factor pf_(fix)′ of thefixed-speed wind turbines.

$\begin{matrix}{{pf}_{fix}^{\prime} = \frac{P_{fix}^{\prime}}{\sqrt{P_{fix}^{\prime 2} + Q_{fix}^{\prime\; 2}}}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

Next, the difference between the power-factor command value for theinterconnection node A and the above overall power factor pf_(fix)′ ofthe fixed-speed wind turbines is calculated.Δpf=pf _(grid) −pf _(fix)′  (9)

To absorb that difference in the variable-speed wind turbines, Δpf isset as a command value correction level, and a value obtained by addingthe command value correction level Δpf to the above power-factor commandvalue pf_(grid) is set as a new power-factor command value. Based on thepower-factor command value, as in the first embodiment described above,the power-factor command values for the individual wind turbines aredetermined using the power factor correction levels Δpf₁, Δpf₂, . . . ,Δpf_(i) corresponding to the individual variable-speed wind turbinesWTG1, WTG2, . . . , WTGi, and the corrected power-factor command valuesare transmitted to the respective wind turbines.

As described above, if fixed-speed wind turbines and variable-speed windturbines are both present, because the wind-power generation system andthe operation control method therefor according to this embodimentdetermine power-factor command values for the variable-speed windturbines by taking into account variations in the power factors of thefixed-speed wind turbines, the variable-speed wind turbines can absorbthe variations in power factor due to the fixed-speed wind turbines.This improves the accuracy of power factor control even if fixed-speedwind turbines are included.

In this embodiment, the overall power factor of the fixed-speed windturbines is determined and is used to correct the power-factor commandvalue for the interconnection node A, and the corrected power-factorcommand value is further corrected using the power factor correctionlevels Δpf_(i) set for the individual variable-speed wind turbines;instead, for example, the power-factor command value corrected using theoverall power factor of the fixed-speed wind turbines may be provided asa power-factor command value for the individual variable-speed windturbines. Although in this case variations in power factor due toreactors present between the individual variable-speed wind turbines andthe interconnection node A are not offset, a considerable advantage canbe achieved in that the variations in power factor due to thefixed-speed wind turbines can be offset.

Although the embodiments of the present invention have been describedabove in detail with reference to the drawings, specific configurationsare not limited to those of the embodiments; design changes etc. areencompassed without departing from the spirit of the present invention.

For example, although the central controller 10 corrects thepower-factor command value in the embodiments described above, thepower-factor command value may instead be corrected, for example, in theindividual wind turbines. In this case, a uniform power-factor commandvalue is transmitted from the central controller 10 to the individualwind turbines, and the power-factor command value received from thecentral controller 10 is corrected in the individual wind turbines usingthe respective power factor correction levels possessed by theindividual wind turbines.

In addition, although the corrected power-factor command values etc. aretransmitted from the central controller 10 by communication in thisembodiment, a configuration may be employed in which, for example, theoperator manually inputs and sets the power-factor command values to therespective wind turbines.

1. An operation control method for a wind-power generation systemcomprising a plurality of variable-speed wind turbines, a plurality offixed-speed wind turbines, and a central controller for providingcontrol commands to the individual wind turbines, outputs of thevariable-speed wind turbines and the fixed-speed wind turbines beingsupplied to a utility grid via a common interconnection node, theoperation method comprising: calculating overall power factor of thefixed-speed wind turbines at the interconnection node; calculating adifference between the calculated overall power factor and apredetermined power-factor command value for the interconnection node;correcting the predetermined power-factor command value using thecalculated difference; and determining power-factor command values forthe individual variable-speed wind turbines based on the correctedpredetermined power-factor command value.
 2. The operation controlmethod for the wind-power generation system according to claim 1,wherein the power-factor command values corresponding to the individualvariable-speed wind turbines are determined by correcting the correctedpredetermined power-factor command value using power factor correctionlevels set for the individual variable-speed wind turbines.
 3. Theoperation control method for the wind-power generation system accordingto claim 2, wherein the power factor correction levels corresponding tothe individual variable-speed wind turbines are determined based onreactance components present between the individual variable-speed windturbines and the interconnection node.
 4. A wind-power generation systemcomprising a plurality of variable-speed wind turbines, a plurality offixed-speed wind turbines, and a central controller for providingcontrol commands to the individual wind turbines, outputs of thevariable-speed wind turbines and the fixed-speed wind turbines beingsupplied to a utility grid via a common interconnection node, wherein,the central controller calculates overall power factor of thefixed-speed wind turbines at the interconnection node, calculates adifference between the calculated power factor and a predeterminedpower-factor command value for the interconnection node, corrects thepredetermined power-factor command value using the calculateddifference, and determines power-factor command values for theindividual variable-speed wind turbines based on the correctedpredetermined power-factor command value.
 5. The wind-power generationsystem according to claim 4, wherein the power-factor command valuescorresponding to the individual variable-speed wind turbines aredetermined by correcting the corrected predetermined power-factorcommand value using power-factor correction levels set for theindividual variable-speed wind turbines.
 6. The wind-power generationsystem according to claim 5, wherein the power factor correction levelscorresponding to the individual variable-speed wind turbines aredetermined based on reactance components present between the individualvariable-speed wind turbines and the interconnection node.